Here's why:

* Polarity arises from uneven sharing of electrons. Covalent bonds occur when atoms share electrons. If the electrons are shared equally, the bond is nonpolar. If one atom pulls the shared electrons closer to itself, the bond becomes polar.

* Electronegativity determines polarity. Electronegativity is the ability of an atom to attract electrons. The greater the difference in electronegativity between two bonded atoms, the more polar the bond.

* Molecular geometry also plays a role. Even if a molecule has polar bonds, the overall molecule can be nonpolar if the polar bonds are arranged symmetrically, canceling out the dipole moments.

Examples:

* Nonpolar covalent molecules:

* CO2 (carbon dioxide): The two C=O bonds are polar, but the linear shape of the molecule means the polarities cancel out.

* CH4 (methane): The C-H bonds are slightly polar, but the tetrahedral shape of the molecule makes it nonpolar.

* Polar covalent molecules:

* H2O (water): The O-H bonds are highly polar, and the bent shape of the molecule means the polarities do not cancel out.

* NH3 (ammonia): The N-H bonds are polar, and the pyramidal shape of the molecule makes it polar.

In summary, a covalent bond is a requirement for polarity, but it's not the only factor. The electronegativity difference between atoms and the molecular geometry all contribute to determining whether a molecule is polar or nonpolar.